Mineralization of articular cartilage in the Sprague-Dawley rat: characterization and mechanical analysis

Abstract

The formation of mineralized deposits in human articular cartilage is a common occurrence [–4]; however, the relationship between mineral deposition and material properties of the articular cartilage is not well understood nor the relationship between mineral deposition and the development of degenerative joint disease. Several different crystalline structures have been identified in articular cartilage and synovial fluid including monosodium urate, calcium pyrophosphate dihydrate (CPPD), and basic calcium phosphates (BCPs). These distinct mineral phases are associated with specific pathologies and mechanisms of crystal formation such as the development of monosodium urate in gout and CPPD in pseudogout. Less is known regarding the deposition of BCPs, a class of compounds including carbonate-substituted hydroxyapatite (cHA), tricalcium phosphates (TCP), octacalcium phosphate (OCP), and whitlockite, in articular cartilage. The presence of BCP calcification of articular cartilage in humans has been associated with decreased joint function [1, 3], aging [2] and severity of osteoarthritis [1, 3]. Commonly used methods of crystal detection such as polarized light microscopy of synovial fluid and conventional radiography of the joint can be insensitive to the detection of BCP crystals and more sensitive techniques such as microradiography or electron microscopy of articular cartilage sections are required to detect areas of BCP mineralization [3, 5, 6]. It is not yet known how regions of mineralization may influence the tribological properties (friction, wear, and lubrication) of the articulating surfaces and the material and structural properties of articular cartilage. Animal models with which to study the mechanisms of mineralization of articular cartilage are limited.

Figure 1

A) Superior view of rat tibial plateau showing isosurface of reconstructed μCT image and areas of mineralization within the articular cartilage (◀), B) Photograph of the same specimen in which areas of well-developed mineralizations appear as white spots on the articular surface (◀), C) Representative backscattered electron image of a coronal section of the medial compartment of the tibia plateau illustrating the articular surface (⧧), mineralized region of articular cartilage (→) and underlying subchondral bone, D) Magnified image of mineralized region depicted in Panel C illustrating the mineralized area (*) and underlying calcified cartilage (#). Numbered points illustrate some of points at which energy dispersive spectra were collected, E) Representative energy dispersive spectra of two mineralized particles illustrating the main components of the mineralized regions are phosphorus (P), calcium (Ca) and oxygen (O) with minor traces of magnesium (Mg) and sodium (Na).

Figure 2

Coronal osteochondral sections showing representative cyst-like regions (◀) in the articular cartilage, which were mineralized prior to histological processing, stained with Hematoxylin and Eosin (A–D) and corresponding serial section below stained with Safranin O and Fast Green (E–H). The substructure within these regions represent vestiges where large mineral deposits resided and coincide with mineralized regions as observed by μCT and surface white spots (Fig 1A–B). Little Safranin O staining was observed within cysts.